DC to AC inverters can be used to generate a multiphase output from a DC power source. For instance, DC to AC inverters can be used in aviation applications to generate a three-phase AC output at a fundamental frequency of about 400 Hz from a DC power source. DC to AC inverters can include a plurality of bridge circuits that include semiconductor switching elements (e.g. IGBTs, SiC transistors, etc.) that are controlled using pulse width modulation techniques to convert a DC input to, for instance, a three-phase output. The inverter can additionally provide a neutral output by adding an additional bridge circuit to form a neutral leg of the inverter. The DC to AC inverter can be used to power single phase loads by coupling loads to one of the output phases and to the neutral output of the inverter.
Powering single phase loads from a multiphase DC to AC inverter can result in unbalanced loading of the inverter. Unbalanced loading can lead to creation of negative sequence and zero sequence currents by the inverter. These currents can create the same negative sequence and zero sequence components in the output voltage of the inverter. In addition, six-pulse inverters can create harmonics at, for instance, negative fifth harmonic and seventh harmonic of the fundamental frequency. Twelve-pulse inverters can create harmonics at, for instance, the negative eleventh harmonic and thirteenth harmonic of the fundamental frequency.
DC to AC inverters can be controlled using various control schemes, including natural frame (abc) control schemes, stationary reference frame control schemes, or synchronous reference frame control schemes. In a natural frame control scheme, identical control structures are used for each phase of the inverter. The control structure for each phase can include a voltage controller and a current controller. The voltage controller can be an outer control loop relative to the current controller and can generate a current reference for the current controller. The current controller can generate a voltage command, which can be used to determine gate timing signals for driving the switching elements of the bridge circuit. If the inverter includes a neutral leg, the voltage command for the neutral leg bridge circuit can be an average value of the voltage command for each of the phases of the inverter. The neutral leg typically does not have its own controller.
The controllers in a natural frame control scheme essentially operate at DC. As a result the controllers have infinite gain at DC and finite gain at the fundamental frequency, the negative sequence, the negative fifth harmonic, the positive seventh harmonic, and other harmonics of interest. FIG. 1 depicts a graphical representation 50 of the gain of the natural frame controller. FIG. 1 plots frequency along the horizontal axis and gain along the vertical axis. As shown in FIG. 1, the controller can reduce the magnitude of the fundamental frequency, negative sequence, negative fifth harmonic, positive seventh harmonic, etc. However, the controller does not eliminate these components as the controller does not have infinite gain at these frequencies. Accordingly, whatever remains of these components in the output voltage can lead to an increase in total harmonic distortion and voltage unbalance in the output voltage of the inverter.
Synchronous reference frame control schemes can operate based on a d-q transformation (e.g., performed using a Park transformation) that transforms the output voltage and current waveforms into a reference frame that rotates synchronously with the output voltage. Multiple synchronous reference frame controllers can include a plurality of control structures to introduce infinite gain at each of the frequencies of interest (e.g., fundamental, −11, −5, +7 and +13 of the fundamental frequency).
However, a condition of synchronous reference frame control schemes is that the controllers in the control structure for each frequency of interest have gains that cross each other below 0 dB. As a result the bandwidth of each controller used in the synchronous reference frame control scheme can be limited, leading to gaps in the control action of each controller. FIG. 2 depicts a graphical representation of the gains of various current and voltage controllers at each of the frequencies of interest for a known synchronous reference frame control scheme 60. FIG. 2 plots frequency along the horizontal axis and gain along the vertical axis. The solid lines represent gains associated with the current controllers. The dashed lines represent gains associated with the voltage controllers. As shown, the synchronous reference frame control scheme can provide infinite gains at the fundamental frequency as well as at selected frequencies of interest. However, due to the limited bandwidth of the controllers, the negative sequence attenuation (e.g., at the −1 frequency) is still required to be performed with the control structure used to attenuate the fundamental frequency. As a result, the control structure does not provide infinite gain for the negative sequence.
FIG. 3 provides a graphical depiction 70 of the gains associated with the regulation of the zero sequence component. FIG. 3 plots frequency along the horizontal axis and gain along the vertical axis. The solid lines represent gains associated with current controllers. The dashed lines represent gains associated with voltage controllers. The zero sequence component does not rotate, and thus it is not controlled using a synchronous reference controller. Rather, the zero sequence is controlled using controllers with finite gains at the fundamental frequency. As a result, typical multiple synchronous reference frame controllers do not completely regulate out the zero sequence and negative sequence components, leading to increased total harmonic distortion and voltage unbalance in the output voltage.
Thus, a need exists for a control scheme for regulating a DC to AC inverter that can reduce output voltage total harmonic distortion and voltage unbalance by providing improved regulation of the zero sequence and negative sequence components generated, for instance, by powering single phase loads with the DC to AC inverter.